THE CARDIAC NERVES. 97 



vapour present to prevent evaporation. If the heart be re-introduced 

 into O it begins to beat again. [An excised heart suspended in 

 ordinary air beats three to four times as long as a heart which is 

 placed upon a glass-plate.] A heart which has ceased to contract 

 spontaneously may contract when an electrical stimulus is applied to 

 it, but it does not do so for a longer time than other muscles (Budge). 



56, Innervation of the Heart, 



[When the heart is removed from the body, or when all the nerves 

 which pass to it are divided, it still beats for some time, so that its 

 movements must depend upon some mechanism situated within itself. 

 The ordinary rhythmical movements of the heart are undoubtedly 

 associated with the presence of nerve ganglia, which exist in the 

 substance of the heart the intracardiac ganglia. But the movements 

 of the heart are influenced by nervous impulses which reach it from 

 without, so that there falls to be studied an intracardiac and an extra- 

 cardiac nervous mechanism.] 



57. The Cardiac Nerves. 



The cardiac plexus is composed of the following nerves (1.) The 

 cardiac branches of the vagus, the branch of the same name from the 

 external branch of the superior laryngeal, a branch from the inferior 

 laryngeal, and sometimes branches from the pulmonary plexus of the 

 vagus (more numerous on the right side). (2.) The superior, middle, 

 inferior, and lowest cardiac branches of the three cervical ganglia and 

 the first thoracic ganglia of the sympathetic. (3.) The inconstant 

 twig of the descending branch of the hypoglossal nerve, which, 

 according to Luschka, arises from the upper cervical ganglia. From 

 the plexus there proceed the deep and the superficial nerves (the 

 latter usually at the division of the pulmonary artery under the arch of 

 the aorta, and containing a ganglion). The following nerves may be 

 separately traced from the plexus 



(.) The plexus coronarius dexter and sinister (Scarpa), which con- 

 tains the vaso-motor nerves for these vessels (physiological proof still 

 wanting) as well as the nerves (sensory?) proceeding from them (to 

 the pericardium 1) 



(b.) Intra-cardiac Nerves and Ganglia. The nerves lying in the 

 grooves of the heart and in its substance, containing numerous ganglia 

 (Remak), which are regarded as the automatic motor centres of the 

 heart. A nervous ring containing numerous ganglia corresponds to the 

 margin of the septum atriorum; there is another in the auriculo- 

 ventricular groove. Where the two meet, they exchange fibres. The 

 ganglia usually lie near the pericardium. In mammals the two largest 



